The present invention relates to a magnetron which is equipped in a microwave oven or the like for generating microwaves.
A conventional cathode arrangement of a magnetron for use in a microwave oven as described, for instance, in Japanese Unexamined Patent Publication No. 46732/1991 (HO1J 23/05) is such that a pair of cathode leads for supporting a cathode pierce through through holes of a stem insulator whereupon they are fixedly attached to the stem insulator via external terminals by brazing. The external terminals not only function as electrodes for energizing the cathode but also serve as vacuum sealing parts for airtight connection with the stem insulator. While a generally employed material for the cathode leads is molybdenum having a high melting point and a high hardness, in case molybdenum is to be brazed with a metallic alloy or the like, a brazing material will not uniformly flow to portions at which molybdenum contacts with the metallic alloy, so that the brazing performance is improved by plating the molybdenum surface contacting with the external terminals with nickel or the like.
However, since connection between the external terminals and the cathode leads is achieved only on a section in thickness direction of the material for the external terminals in such an arrangement of a magnetron, the contacting area will be extremely small, so that there were presented drawbacks that through holes were generated through progression of oxidation of the brazing material and that vacuum break was caused at an early stage at connecting portions between the cathode leads and the external terminals.
It is an object of the present invention to provide a magnetron of high reliability in which vacuum break does hardly occur at connecting portions between the cathode leads and the external terminals.
In accordance with present invention, there is provided a magnetron including a tubular metallic container which is air-tightly connected to an anode to define a part of a vacuum container, cathode leads for supporting a cathode with filaments being disposed in a central axial portion of the anode, a stem insulator formed with through holes through which the cathode leads pass, an open end portion of the tubular metallic container being air-tightly connected to circumference of the stem insulator, and external terminals formed with planar portions which are air-tightly connected to a surface of the stem insulator opposite to a surface facing the tubular metallic container and with connecting portions which are air-tightly connected to the cathode leads. The connecting portions are arranged to be bent in an axial direction of the cathode leads.
According to the above arrangement, the connecting portions are bent in the axial direction of the cathode leads, areas at which the connecting portions and the cathode leads contact with each other are increased. Accordingly, even though oxidation of the air-tightly connected portions is progressed, the distance from end portions of the connecting portions to the through holes through which the cathode leads pass becomes long, so that it is possible to prevent vacuum break caused by oxidation or the like.
Since the connecting portions are so arranged that the cathode leads pierce through cylindrical portions formed on the external terminals to cover an outer periphery of the cathode leads to achieve an air-tight connection between the cylindrical portions and the cathode leads by brazing, the brazed portions between the cathode leads and the external terminals can prevent vacuum break caused by oxidation or the like of the brazing material since the brazed areas in the axial direction of the cathode leads is increased by the cylindrical portions and the distance from the end portions of the cylindrical portions to the through holes is elongated.
Since the cylindrical portions are characterized by covering the end portions of the cathode leads, the brazed portions between the cathode leads and the external terminals are brazed with the brazed areas in the axial direction of the cathode leads being increased and the end portions of the cathode leads being covered by the cylindrical portions. Accordingly, the melted brazing material will not be exposed to the surface, so that it is possible to delay progression of oxidation of the brazing material and thus to prevent vacuum break from occurring at an early stage.
Further, since the cylindrical portions are characterized by being inserted into the interior of the stem insulator, the cathode leads and the cylindrical portions are brazed in the interior of the stem insulator. Accordingly, it will not be necessary to make the cathode leads project from the stem insulator, so that the cathode leads are allowed to be short to thereby reduce manufacturing costs.
Since annular concave portions into which the cylindrical portions are inserted are formed on the stem insulator, the cylindrical portions is supported between the outer peripheral surface of the cathode leads and the wall surface of the annular concave portions, to thereby reliably braze the cylindrical portions and the cathode leads.
Moreover, the cathode leads are so designed that at least a material for forming the portions which are air-tightly connected with the connecting portions is molybdenum. In case molybdenum is brazed with metallic alloy, it has been conventionally required to provide a metallic layer of nickel plating or the like on the surface of the molybdenum since the brazing material could not uniformly flow to portions at which the molybdenum and metallic alloy are contacted. The increase in connecting area in the above arrangement, however, permits brazing without providing a metallic layer on the surface of the molybdenum which is air-tightly connected to the connecting portions, thereby to achieve cuts in costs, involving no costs for providing metallic layers.